Plastids

A chloroplast is internally organised into a system of membranes and a ground substance specialised for photosynthesis. Flattened membranous sacs called thylakoids are arranged in stacks resembling piles of coins, and each such stack is termed a granum, as explained in NCERT Class 11, Chapter 8 (Cell: The Unit of Life); the thylakoid membranes carry chlorophyll and other pigments that trap light energy. The option describing stroma lamellae is incorrect because those are the flat membranous tubules that connect the thylakoids of different grana rather than the light-trapping stacks themselves. The choice naming the stroma is wrong, since the stroma is the colourless fluid matrix where the enzymatic dark reactions of carbon fixation occur. The mention of quantasome ribosomes confuses two ideas, because ribosomes are protein-synthesising particles while quantasomes are pigment assemblies on thylakoids. The stacking of thylakoids into grana enormously enlarges the membrane surface available for the pigment systems, so that more photons can be captured per unit volume of the plastid. Stroma lamellae then interconnect these grana, allowing the products of the light reactions to be shared with the surrounding stroma where sugars are built. A structural consistency check confirms the answer: only the stacked thylakoid units, the grana, both hold the pigments and provide the membrane surface required to capture sunlight for the light reactions.